1. Field of the Invention
The present invention relates to an X-ray CT apparatus and an image processing apparatus which generate an image within a patient, on the basis of projection data obtained by irradiating the patient with X-rays.
2. Description of the Related Art
The progress of X-ray CT apparatuses is remarkable, and in compliance with the eager desire of medical treatment sites to radiograph a patient at a higher precision (higher resolution) and in a wider range, a multislice X-ray CT apparatus has been developed and has come into considerably wide use in recent years. The multislice X-ray CT apparatus is a scanner including an X-ray source which irradiates the patient with fan beam X-rays having a spread width in a slice direction (the lengthwise direction of a patient couch), and a two-dimensional detector of a structure in which a plurality of detection element rows (of 4 rows, 8 rows, 16 rows, or the like) are arrayed in the slice direction, wherein the X-rays are moved by multiscan or helical scan. Thus, as compared with a single-slice X-ray CT apparatus, the multislice X-ray CT apparatus can obtain volume data over a wider range within the patient, at a higher precision and in a shorter time.
In a case where a blood vessel in a CT image generated by such an X-ray CT apparatus has a high-contrast substance (a substance having a high X-ray attenuation coefficient) nearby, the situation of a cavity in the blood vessel becomes unclear under the influence of unsharpness ascribable to the substance. Heretofore, as a method for improving the image quality of a noted region without being influenced by a high-contrast region, there has been known a method wherein image processing is executed after the CT number of the high-contrast region is set at a suitable value adjusted to the CT number of the surroundings, whereupon the value of the high-contrast region is restored. With this method, however, it has been impossible to favorably eliminate the unsharpness of the surroundings of the high-contrast region (refer to JP-A-10-40372).